You can link to a telephone number from text, an image, or a button.*
To start, navigate to the email editor, and add or click on the panel you want your link to appear in.
From here:

As a hyperlink

Highlight the text you would like to link, and then click the hyperlink icon from the text editing toolbar above.

• Under Link Type, choose Telephone.*
• Under Telephone Number, type in the number you want to link to.
• Click Apply

Link from an image

• Add, or click on the panel with the image you'd like to link.
• Click the link icon over the image.
• Under Link Type, choose Telephone.
• Under Telephone Number, type in the number you want to link to.
• Click Apply

Link from a button

• Choose to link from a button within a panel, or a standalone button panel:
  • Text / Text & Images panel - Click the button icon.
  • Button panel - Click on the options link, or double click the button.
• Under Link Type, choose Telephone (1).
• Under Telephone Number, type in the number you want to link to (2).
• Type in the text you would like to appear on your button (3).
• Click Apply

When a contact clicks on the link from your email, it will open their default telephone client.
This will depend on how they are viewing your email, e.g: via mobile, laptop, tablet, desktop.

* You can also link to:

• A web address (including custom field placeholders)
• A pre-filled email
• Social Media links (e.g: LinkedIn)
• A landing page (e.g: Survey)
• A system link (e.g: Unsubscribe link)